Asaoka K, Tesk J A
Department of Dental Engineering, School of Dentistry, Tokushima University, Japan.
J Dent Res. 1990 Feb;69(2):463-9. doi: 10.1177/00220345900690020901.
Porcelain-fused-to-metal (PFM) restorations may develop cracks during processing or in-mouth service if the relative physico-mechanical properties of the porcelain and metal are highly mismatched. Precise conditions when this might occur are not known. Many processing and property variations can affect the stresses developed throughout a porcelain-metal system. To understand this, we conducted a computer simulation of stress developed in a PFM beam. The simulation considers cooling from temperatures higher than the porcelain sagpoint. The following temperature-dependent factors were incorporated: the elastic modulus, shear viscosity (porcelain), and coefficients of thermal expansion. The cooling rate dependencies of the glass transition temperature, (Tg), and the temperature distribution during cooling were also included. The results suggest that transient tensile stress at the porcelain alloy interface may result in cracks in the porcelain during cooling. Occlusal forces may set up stresses to cause cracking at the surface of the porcelain if the compressive residual stress is not high enough. PFM restorations with an alloy of high thermal expansion coefficient require rapid cooling; on the contrary, PFM restorations with the alloys of lower coefficients require slow cooling. A high cooling rate can make up for thermal expansion mismatches between the alloy and the porcelain up to 2 x 10(-6)/degrees C. Finally, the results indicated that curvature was not a sensitive indication of stress for a multimaterial beam when visco-elastic relaxation and high cooling rates are involved. For the case modeled here, curvature varied inversely with a 1/2 to 1/7th power of the stress.
如果瓷和金属的相对物理机械性能高度不匹配，烤瓷熔附金属（PFM）修复体在加工过程中或口腔使用时可能会出现裂纹。这种情况可能发生的确切条件尚不清楚。许多加工和性能变化会影响整个烤瓷 - 金属系统中产生的应力。为了理解这一点，我们对PFM梁中产生的应力进行了计算机模拟。该模拟考虑了从高于瓷垂熔温度的温度开始冷却的过程。纳入了以下与温度相关的因素：弹性模量、剪切粘度（瓷）和热膨胀系数。还包括玻璃化转变温度（Tg）的冷却速率依赖性以及冷却过程中的温度分布。结果表明，在冷却过程中，瓷 - 合金界面处的瞬态拉应力可能导致瓷出现裂纹。如果压缩残余应力不够高，咬合力可能会产生应力，导致瓷表面出现裂纹。具有高热膨胀系数合金的PFM修复体需要快速冷却；相反，具有较低系数合金的PFM修复体需要缓慢冷却。高冷却速率可以弥补合金与瓷之间高达2×10⁻⁶/℃的热膨胀不匹配。最后，结果表明，当涉及粘弹性松弛和高冷却速率时，曲率对于多材料梁来说并不是应力的敏感指标。对于此处建模的情况，曲率与应力的1/2至1/7次幂成反比变化。
